Method for generating a congestion flag based on measured system load

ABSTRACT

A radio base station generates a congestion status flag, based on measured resource usage in its cell, and based on performance of sessions in the cell. The flag may be a one bit, or a small number of bits, indicating whether the base station is congested. The flag can be sent to neighbouring radio base stations, for use in determining whether to perform handovers to that radio base station. The flag generated in a radio base station, and the flags generated in neighbouring radio base stations, can also be sent to user equipment in a cell.

FIELD OF THE INVENTION

The present invention relates to methods and arrangements in cellularmobile communication systems, in particular to efficient radio resourcemanagement in an evolved UMTS Terrestrial Radio Access (E-UTRA) Network.

BACKGROUND AND SUMMARY

In order to meet high target bit rate requirement in an evolved UMTSTerrestrial Radio Access (E-UTRA) Network, efficient radio resourcemanagement (RRM) techniques are required. The realization of efficientRRM methods requires exchange of measurements across different networknodes. The measurement reports indicate the resource usage in a certainnetwork node such as average transmission power level. Generally thesemeasurements are needed before the execution of an RRM function. Sincethere are different types of resources, there are several kinds ofmeasurement reports that are to be communicated between the networknodes, notably between NodeBs, in order to efficiently perform the RRMoperation. Although measurement reports are critically important,excessive or unnecessary measurement reporting should be avoided inorder to minimize the signaling overheads. Our key observation is thatthere are a multitude of radio resource measurements that need to becommunicated between radio network nodes for efficient radio resourcemanagement.

In cellular systems the RRM techniques, namely admission control andhandover control, play a key role in achieving the desired grade andquality of service (GoS/QoS) targets (see also the documents 3GPP TR25.922 “Radio Resource Management Strategies” and 3GPP TS 25.331 “RadioResource Protocol Specifications” issued by the 3rd GenerationPartnership Project). The main purpose of the admission control is toensure that the QoS of the new session can be fulfilled. Secondly, itshould also ensure that the admission of a new user should not adverselyaffect the performance of the ongoing sessions.

In UTRA the admission control functionality is located in the radionetwork controller (RNC), where the radio resource control (RRC)protocol is also terminated. The admission control process in UTRAshould take into account several types of resources such as downlinktransmitted power, downlink channelization code usage and uplinkreceived total wideband power (RTWP). In UTRA, similarly to admissioncontrol, the handover is also carried out by the RNC, which is aware ofthe resource situation of the target cells, as discussed in the document3GPP TR 25.922 “Radio Resource Management Strategies”.

When a new call or a handover is required, it will be denied if therequired resources are not available in the target cell of the targetNode B.

FIG. 1 illustrates a handover attempt failure due to insufficient cellresources. That is, FIG. 1 illustrates a serving Node B sending ahandover request to a target Node B. The handover request is denied bythe target Node B since uplink resource blocks are either fully utilizedor their usage is above a certain threshold. The handover failure canhowever be prevented to some extent by inquiring the resource usage atthe target Node B prior to handover attempt.

FIG. 2 therefore illustrates another scenario, where the serving Node Bfirst requests the target Node B to report its measurements, for alltype of resources, in order to prevent any handover failure due to thecell resource bottleneck. Only after receiving the measurement reportsdoes the serving Node B determine whether that possible target Node Bshould be the target Node B for a handover attempt. It is clear that, inthis type of situation where the target Node B cannot serve a new calldue to lack of resources, the complete set of measurement reporting iswasteful of bandwidth and leads to unnecessary handover latency. In caseof a failure of the handover attempt, the serving Node B typicallyrepeats the above mentioned procedures with the second best target NodeB and so on.

Hence, in an existing system the situation may arise in which severalhandover attempts may be needed before the handover is successfullyexecuted. Secondly this would lead to considerable signaling overheads.The above mentioned problems have been identified and realized alreadyfrom the past experiences in UTRA. The current WCDMA network utilizestry/fail/adjust/reattempt way of handling resource congestions problems.

It has thus been observed to be a problem that admission control andhandover control cannot be efficiently used due to an inefficient use ofradio resources. It is thus an objective to reduce unnecessarymeasurement reports in E-UTRA and a need to specify simple mechanisms ofexchanging measurement reports between the Node Bs. There is also a needfor devising mechanisms whereby the congestion situation in networkcould be communicated between the Node Bs in simple manner. Forinstance, it has been proposed to multicast congestion status in theform of single level of congestion such as available free capacity. Inthe current proposal the congestion status does not consider theaggregation of all different types of radio, transport network andhardware resources. In addition the congestion status does not includethe measured performance such as ongoing grade of service performance.However, a situation can arise in which the system is not fullycongested but still the requested QoS requirements cannot be met.

According to an aspect of the present invention, a network nodegenerates a congestion flag, indicative of measured resource usage andmeasured performance, and this can then be used as a basis fordetermining whether to attempt to access that node.

In preferred embodiments, the invention can therefore provide some ofthe following advantages, namely:

A simple way to indicate congestion status to the neighbour basestations and to the UEs.

Minimizing of handover failure and handover attempt failure, therebyimproving grade of service. Particular advantage in distributed systemswhere handover is located in Node B.

Reduction of measurement reports when they are not needed. Particularadvantage in distributed systems where excessive reports can overwhelmsignalling links between the base stations.

Allowing the serving Node B to update the neighbour cell list. This willallow UE to do measurement on cells, which can accommodate new links.This will also reduce the number of UE measurements.

New call blocking can be reduced. Particular advantage in distributedsystems where admission control is located in Node B.

Realization of multi-cell RAM in distributed architecture where there isno centralized controller with radio related information.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a handover attempt failure due to insufficient cellresources.

FIG. 2 illustrates measurements required to prevent handover failures,in accordance with the prior art.

FIG. 3 illustrates a radio access network architecture of E-UTRA.

FIG. 4 illustrates an example of the resource situation in a target NodeB.

FIG. 5 illustrates an example for a flag generation at a Node B.

FIG. 6 illustrates a 4-level congestion status diagram realized bysingle bit using state transition method.

FIG. 7 is a flow chat, illustrating a process for generating acongestion status flag.

FIG. 8 is a flow chart, illustrating a process for initiating a handoverusing a congestion status flag.

FIG. 9 is a flow chart, illustrating the transmission of the updatedneighbour cell list to a user equipment.

FIG. 10 illustrates a part of a telecommunication network including aradio base station and a user equipment for which the present inventionis applied.

DESCRIPTION

The present invention is particularly applicable to a cellular wirelesscommunications system, for example as shown in FIG. 3. In the examplenetwork shown in FIG. 3, the network 10 is an evolved UMTS TerrestrialRadio Access (E-UTRA) network, although it will be apparent that theinvention is similar applicable to other types of network.

FIG. 1 shows a part of the network, containing four radio base stations,or Node Bs, although it will be apparent that the network will containmany more than this number. In E-UTRA, the RRC terminates in the Node B.As shown in FIG. 3, the Node Bs each have respective connections 12, 14,etc, to an access gateway (aGW) 16, which performs only user planeswitching but is not aware of any radio related information such as cellload etc. In order to carry out admission control and handovers, theserving network node should be aware of the load at the target node.Thus in E-UTRA both admission control and handovers are placed in theNode B. Each of the direct interfaces 18, 20, etc between the Node Bs(the X2 interfaces %, is simply a control plane that can transportmeasurement reports, handover related commands etc.

E-UTRA is designed to achieve a significantly higher data rare comparedto UTRA. Secondly, in E-UTRA the latency requirements of certainfunctions such as handover, is considerably more stringent than inWCDMA. This means that admission control and handover failures should beminimized because otherwise latency will increase. Thus, in E-UTRA,efficient admission control and handover execution may require the NodeB to monitor various types of measurements such as transmitted power,RTWP, resource block usage, transport network channel usage etc. Byusing more comprehensive resource information at the Node B, theadmission and handover decisions can be made more predictable.

In E-UTRA there are several types of radio and network resources thatare assigned to a session by the serving Node B. Some importantresources that are to be used in E-UTRA are: the Downlink transmitpower; the Uplink received total Interference; the Downlink resourceblocks (DL RB); the Uplink resource blocks (UL RB); the Uplink transportnetwork resources (UL TrNetCh); the Downlink transport network resources(DL TrNetCh); and the Hardware resources.

It should be noted that a new call or a handover request can be denieddue to the lack of availability of any one of the cell resources listedabove. This is illustrated in FIG. 4, which shows certain average usagelevels of different types of cell resources in a target cell of thetarget Node B. FIG. 4 illustrates an example situation, where only oneof the cell resources (that is, the uplink resource blocks) is fullyoccupied, but other resources are still available.

It is the basic idea of the current invention that each Node B generatesa congestion status flag based on resource usage and measuredperformance to indicate whether or not a new connection can be served.The flag can be sent to neighbour Node Bs to prevent handover failuresor it can be broadcasted in its own cell to facilitate admissioncontrol. The congestion status flag can also be used by the serving NodeB to dynamically set the neighbour cell list, which is used by the UE todo downlink measurements.

FIG. 5 illustrates the generation of the flag in the Node B. Two sets ofmeasurements are fed into the flag generating algorithm as shown in FIG.5. The first set of measurements relate to usage of resources at theNode B such as the downlink transmitted power (X₁), the downlinkresource block usage (X₂), the uplink transport network channel usage(X_(N)), etc. The second set of measurements relate to the measuredperformance, which may include the fraction of unsatisfied users (Y₁),the new session blocking probability or P_(b) (Y₂), the ongoing sessiondropping probability or P_(d) (Y_(M)), the mean user bit rate etc. Themeasurements can be done over a certain time period. Different resourceand performance metrics types may have different measurement periods.

As shown in FIG. 5, each measurement is compared with a certainthreshold. Specifically, as indicated in the figure, X_(i) and γ_(i)represent measured cell resource i and the corresponding threshold,respectively. Similarly, Y_(i) and β_(i) represent measured performancei and the corresponding threshold, respectively. In this example, any ofthe input measurements exceeding its respective threshold would generatea flag=1, indicating congestion in the network. The algorithm thatgenerates the flag in the Node B is implementation dependent (involvingthe setting of threshold levels, defining the flag generation logic,etc). However, the main idea is the generation of congestion flag basedon two sets of measurements: resource usage and measurement performance.

In FIG. 5 the flag generation logic is exemplified by a simple ORoperation. Other flag generation mechanisms assigning weights todifferent resources and hysteresis values are also possible.

In its simplest form, the flag may comprise a single bit of informationas depicted in the table below, where Flag=1 and Flag=0 manifestcongestion and no congestion situations respectively in a cell.

TABLE 1 Preventive flag Flag Meaning 1 Congestion 0 NO Congestion

Different services typically require different amounts of resources.Thus, it is advantageous to have multilevel signalling, i.e. someintermediate levels between congestion and no congestion states. Forexample, multilevel congestion status can be realized either by usingmore bits (e.g. 2 bits for 4-levels); or by using multiple states andstate transitions with the same number of bits (e.g. 1 bit for4-levels).

As an example, we can consider the 4-level case, where the congestionstatus, in addition to the two extreme situations, can also indicate ifthe target cell can accept low or medium bit rate services. The tablebelow depicts as examples two 4-level congestion status signallingmechanisms, using 2-bits and using a single bit to indicate statetransitions, respectively.

Congestion Flag Method 2: Method 1: State transition 2-bits (states)Meaning 00 1 NO congestion 01 2 Accept medium bit rate 10 3 Accept lowbit rate 11 4 Congestion

The state transition based signalling mechanism for 4-level congestionstatus is also illustrated by the 4-state transition diagram in FIG. 6.The state transition method, i.e. method 2 in the table, governs thetransitions between congestion states rather than indicating thecongestion level itself. Thereby multiple congestion levels aremaintained but the signalled information between network nodes is 1 bitat a time. However the obvious drawback is that only adjacent congestionstates can be reported using 1 bit. On the other hand, in the firstmethod, whereby each state is explicitly represented by 2-bits, acongestion status report can be completely independent of the previousreport. An M-level congestion status flag generation algorithm in theNode B can be realized by having M−1 thresholds for both sets of inputparameters: resource usage and measurement performance. Thus, a 4-levelcongestion status would require 3 thresholds for each type ofmeasurement.

The E-UTRA system shall also support inter-RAT handovers, which meanshandovers to other access technologies such as UTRA and GERAN. In caseNode Bs or base stations belonging to different radio accesstechnologies are co-sited (e.g. UTRA/E-UTRA Node Bs at same location), amulti-RAT flag can also be sent to the neighbour Node Bs of these otherRATs. This will provide E-UTRA Node Bs with an overall status of theresource situation in cells belonging to other technologies. Themulti-RAT flag can depict 2-level or multilevel congestion status asdescribed above. The table below illustrates an example, where 2-levelcongestion status indicates the congestion in different technologies. Inthis example both E-UTRA Node B (eNode B) and GSM base station arecongested.

eNode B ID Node B ID GSM BS ID 1 0 1

FIG. 7 is thus a flow chart, illustrating a method in accordance withthe invention, whereby a congestion status flag is generated. In step 70of the process, measurements are made, relating to the congestion statusof the relevant radio base station. As mentioned above, these caninclude any relevant measurements relating to the downlink or uplinkradio or fixed network resources. In step 72, measurements are made,relating to the performance of the base station. As described above,these can include any relevant measurements relating to the performanceof the ongoing sessions in a cell. In step 74, a congestion status flagis generated. As described above, the flag can be generated by comparingthe relevant measurements with respective thresholds, and thengenerating the flag to indicate the congestion status of the basestation. In step 76, the radio base station transmits the generatedcongestion status flag to other network entities.

More specifically, regarding step 76, one possibility is that thecongestion status flag can be used to prevent handover failures. Thus,each Node B sends the congestion flag to its neighbor Node Bs. The flagwill indicate the overall resource situation in the transmitting Node B.This means that, when the transmitting Node B is a potential target NodeB for a handover, the serving Node B can be prevented from sendinghandover requests to that target Node B when it is congested, i.e. whenits flag=1. On the one hand this will reduce the handover failureattempt, thereby minimizing handover latency and preventing unnecessarysignaling exchange. On the other hand this will allow the serving Node Bto select the next possible Node B in terms of resource availability,thereby minimizing the call dropping probability. By using multi-RATcongestion status the handover failures due to congestion of basestations belonging to other access technologies can also be minimized.

The simplest way to send the flag is via the control plane, that is, viathe respective Node B-Node B interface 18, 20, etc as shown in FIG. 3,in E-UTRA. It is important to note that the congestion flag can eitherbe sent as a separate message or it can be concatenated to other typesof signaling messages. The main advantage of the former scheme is thatsome overheads such as base station identifier can be reduced. In anycase the target base station receiving the congestion status flag shouldbe able to uniquely identify and interpret the meaning of the congestionflag.

Another option is to send the flag by piggybacking it with the user dataover the user plane and routing it to the destination Node Bs via theaccess gateway (aGW) 16. This approach is realizable since the flagcomprises only one, or very few, bits. The control plane (Node B-Node Binterface) is expected to have much lower capacity than the user plane.Thus, the second approach is also useful in situations where there iscongestion on the Node B-Node B control plane.

FIG. 8 is a flow chart, illustrating a process for initiating ahandover, using the congestion status flag as described above. Thus, instep 90, the serving base station identifies another Node B as apotential target base station for a handover. In step 92, the servingbase station reads the congestion status flag sent by that potentialtarget base station and, in step 94, it determines whether thatcongestion status flag indicates that the potential target base stationis congested. If the flag indicates that the potential target basestation is congested, the process returns to step 90, and the servingbase station identifies an alternative potential target base station. Ifthe flag indicates that the potential target base station is notcongested, the process passes to step 96, and the serving base stationproceeds with the handover to that target base station.

The indication of congestion status does not prevent any Node B fromrequesting measurement reports from neighbouring Node Bs, whosecongestion status flags indicate that they are congested. However, aNode B can have an option not to request measurements reports fromcongested neighbour Node BS. This will reduce unnecessary signallingoverheads and processing at the Node B.

The congestion status flag can also be used for dynamic setting ofneighbour cell lists. As shown in FIG. 9, in step 100 of that process, aserving Node B can receive the congestion status information reported byone or more of the neighbour Node Bs, as described above. The servingNode B can then update its existing neighbour cell list at step 102 byexcluding from that list any neighbouring Node B whose flag=1. Theupdated list can then be sent to the users in that cell by the servingNode B. This will prevent those users from doing measurements (forhandover or cell change) on cells that are congested.

Thus, FIG. 10 shows a system including a radio base station (RBS) 81,including a block 811 for generating a congestion status flag, operatingas described above. FIG. 10 also shows a user equipment (UE) 82, whichmay take the form of a mobile phone, or any type of portable wirelesscommunications device, including a block 821 for reading messagesreceived from the RBS 81 and a processor 822.

In the process of FIG. 9, the RBS 81 transmits the updated neighbourcell list to the UE 82, which is able to use that list for futurehandovers or cell changes.

The congestion status flag can also be used for prevention of admissionfailures. Thus, considering the system of FIG. 10, the flag can also bebroadcast to the users in its own cell using the same signallingmechanism as described above. Moreover, since the serving Node B 81 isalso aware of the congestion status in the neighbour Node Bs, theserving Node B 81 can also broadcast the flag status of the neighbourNode BS. This will prevent the UE 82 from listening to the neighbouringNode Bs for the purpose of inferring the congestion status.

In order to prioritize handover over new calls or vice verse, differentmeasurement thresholds can be used to generate the flag for handoversand for admission control. The UE can then consider the overall resourcesituation by reading the broadcast flag before initiating a call (e.g.before sending random access request). For example, the UE can avoidaccess to a congested cell (e.g. whose flag=1) even though it is thebest in terms of coverage or quality. Rather, the UE shall access thenext best cell in terms of coverage or quality, which is not congested(e.g. flag=0). The multi-RAT flag can be used to steer the UE to accessthe best cell belonging to the best possible access technology in termsof available resources.

In the case of UE based handovers (e.g. in fully distributed systems),the broadcast information can also be used by the user equipment toperform handovers.

There is thus described a system for generating congestion statusinformation, which can be used by network entities to operateefficiently.

1. A method of indicating a status of a radio base station of atelecommunication network wherein said method comprises generating acongestion status flag based on a measured resource usage and based on ameasured performance of said radio base station.
 2. The method accordingto claim 1 wherein the measured resource usage relates to usage of a setof any downlink or uplink radio or fixed network resources in the radiobase station.
 3. The method according to claim 1 wherein the measuredperformance relates to performance of ongoing sessions in a cell.
 4. Themethod according to claim 1 wherein the congestion status flag isgenerated by comparing the measured resource usage and the measuredperformance with respective thresholds.
 5. The method according to claim1 wherein the flag has two possible values, and wherein one valueindicates full congestion whereas the other value indicates nocongestion.
 6. The method according to claim 1 wherein the flag has morethan two possible values, and wherein highest and lowest of said valuesindicate full congestion and no congestion, respectively, andintermediate values indicate respective different intermediate levels ofcongestion.
 7. The method according to claim 6 wherein a congestionstatus having more than two values is signalled by a single bit usingstate transitions.
 8. The method according to claim 7 wherein thecongestion status of the radio base station belonging to multiple accesstechnologies is signalled.
 9. The method according to claim 1,comprising sending said congestion status flag to at least one otherradio base station.
 10. The method according to claim 9, wherein thecongestion status is signaled as a separate and independent message. 11.The method according to claim 9, wherein the congestion status is sentby concatenating it with other signaling messages.
 12. The methodaccording to claim 9, wherein the congestion status is sent byconcatenating or piggybacking it with the user data.
 13. The methodaccording to claim 1, comprising sending said congestion status flag tousers in its own cell.
 14. The method according to claim 13, furthercomprising sending to users in its own cell said congestion status flagrelating to at least one other radio base station.
 15. A method ofoperation of a radio base station of a telecommunication network whereinsaid method comprises reading a congestion status flag generated in atleast one other radio base station, said congestion status flag beingbased on a measured resource usage and based on a measured performanceof said at least one other radio base station.
 16. The method accordingto claim 15, wherein said radio base station, acting as a serving radiobase station, utilizes said congestion status flag to determine a targetradio base station for the purpose of handovers.
 17. The methodaccording to claim 15, wherein said radio base station, acting as aserving radio base station, utilizes said congestion status flag todynamically update neighbour cell lists used by user equipment toperform downlink measurements for handovers and cell reselection.
 18. Amethod of operation of a user equipment in a telecommunication networkincluding one or more radio base stations, the method comprising readinga congestion status flag generated in a radio base station, saidcongestion status flag being based on a measured resource usage andbased on a measured performance of said radio base station.
 19. A methodaccording to claim 18, comprising reading said congestion status flag ofits own cell and at least one neighbouring cell, and utilizing thisinformation to decide the best radio base station in terms of cellresources to establish a new call.
 20. The method according to claim 19,comprising utilizing said congestion status information to decide thebest radio base station in terms of cell resources to perform handover.21. A radio base station for a telecommunication network wherein saidradio base station comprises a means for generating a congestion statusflag based on the measured resource usage and measured performance. 22.A user equipment for a telecommunication network including one or moreradio base stations, wherein the user equipment comprises a means forreading a congestion status message based on the measured resource usageand measured performance of its own cell and at least one neighbouringcell, and a means for determining the best radio base station in termsof cell resources to establish a new call based on this information.